1. Field of the Invention
The present invention relates to internal combustion engines and, more particularly, to a high-power two-stroke engine for use in model construction.
2. Description of Related Art
Compact high-power two-stroke engines (for example “nitro engines” with a carburetor and glow plugs, or gasoline engines with spark plugs), which output power levels of up to several PS (for example 2.6 PS) at speeds of up to 40,000 rpm, are used primarily in applications with space and/or weight restrictions, for example in go-karts, ultra-light motorized aircraft (ULM), microlight aircraft (Micro Air Vehicles MAV) and the like. They have in particular for a long time proven themselves, in addition to electrical high-power drives, for driving model aircraft, model boats and model cars, specifically in the demanding hobby field and for racing applications.
The exemplary construction of a high-power two-stroke engine having one cylinder is shown in an exploded illustration in FIG. 1a. The carburetor has been omitted here for clarity. The high-power two-stroke engine 10 comprises a crankcase 11 having an integrally formed cylinder 12 which has cooling fins. A crankshaft 18 is rotatably mounted in the crankcase 11, so as to lie horizontal, by means of two ball bearings 14 and 16. Non-ferrous metal plain bearings could also be used instead of the ball bearings 14, 16 for mounting the crankshaft 18. The crankshaft 18 projects out of the crankcase 11 through the front ball bearing 14 and is secured, for example, by means of a conical driver 13. Power is taken from the projecting end of the crankshaft 18. At the rear side, the crankcase 11 is closed off by means of a cover 20 which is screwed by means of screws 21 to the crankcase 11 and closes off the latter in a gas-tight manner with the aid of an interposed seal 19. The crankshaft 18 supports, at the rear, circular-disk-shaped end (section 36 in FIG. 2a), an eccentrically arranged crank pin 40 on which a connecting rod 22 is rotatably seated with its lower end. Formed opposite the crank pin 40 is a counterweight 39. The upper end of the connecting rod 22 is rotatably connected, by means of a piston pin 26 which is secured by means of securing rings 23, to a piston 24 which can move in a reciprocating fashion so as to slide in a vertical running sleeve 25 which is inserted in the cylinder 12. The upwardly open cylinder 12 is closed off by means of a head plate 28 which, in the centre, has a threaded bore for screwing in a glow plug 30. The head plate 28 is fastened to the cylinder 12 by means of a cooling head 29 which sits on top of said head plate 28 and is screwed by means of screws 31 to the cylinder 12 through a crown of vertical bores (or to the crankcase 11 through the cylinder). In order to adjust the spacing of the head plate 28 from the running sleeve 25 and therefore to adjust the working space, annular adjusting plates of a predetermined thickness are arranged between the running sleeve 25 and the head plate.
In the high-power two-stroke engine 10 of the type illustrated in FIG. 1a, the intake of the air/fuel mixture and the discharge of the combustion gases are controlled in a manner known per se by means of the piston 24 which runs in the running sleeve 25 (and by means of the crankshaft 18). During an upward movement of the piston 24, a vacuum is generated in the space which is situated below the piston 24 and is connected to the crank space (44 in FIG. 4) of the crankcase 11, which vacuum sucks air/fuel mixture via an intake pipe 15 on the crankcase 11 from the carburetor which is attached there. As the piston 24 moves downward again after passing through top dead center TDC and an ignition, it firstly opens an exhaust gas outlet opening 17 which is arranged laterally on the cylinder 12 and via which the combustion gases are discharged. The connection between the intake pipe 15 and the crank space 44 of the crankcase 11 is interrupted, and the air/fuel mixture situated in the interior 44 is then pushed via transfer ducts 32a-32c in FIG. 1b), which are arranged laterally in the cylinder wall, into the working space, which is freed up by the piston 24, above the piston 24, where it pushes out the combustion gases (charge exchange). During the renewed downward movement of the piston 24, the mixture is compressed and ignited after passing through TDC. The further details of said two-stroke process are familiar to a person skilled in the art and are therefore not discussed in any more detail here.
It is pointed out at this stage that, in the two-stroke high-power engine 10 illustrated in FIG. 1a, the exhaust gas outlet opening 17 is arranged directly opposite the intake pipe 15, and that in FIG. 1b, a total of three equivalent transfer ducts 32a-32c are provided which, in relation to the exhaust gas outlet opening 17, are arranged offset with respect to the cylinder axis by 90°, 180° and 270°. The transfer ducts 32a-32c are generated by the interaction of recesses in the cylinder wall with the running sleeve 25 (FIG. 1a) which is inserted in the cylinder 12. The arrangement of the three transfer ducts 32a-32c corresponds to a highly effective combined transverse-flow/reverse-flow scavenging arrangement, as described in EP-A1-0 059 872. Correspondingly, 4 horizontal slots are provided in the running sleeve 25—as can be seen in FIG. la—which slots are arranged offset relative to one another by 90° in each case. The one slot assigned to the exhaust gas outlet opening 17 is situated here at a different level than the three other transfer slots.
The intake of the gas/fuel mixture, as is known per se, via the crankshaft 18 and its control by means of the crankshaft 18 (in this regard, see for example DE-U1-295 11 007) results from the design of the crankshaft 18 as per FIGS. 2a, 2b, 3a and 3b and its interaction with the crankcase 11 as per FIGS. 4 and 5. The crankshaft 18 is divided along the axis 42 into several sections 33, . . . , 36 of different outer diameter. The frontmost section 33 projects out of the crankcase 11 and serves for the take-off of engine power. The crankshaft 18 is mounted with the next section 34 in the front ball bearing 14. The thread 41 arranged between the two sections 33 and 34 serves to fasten parts to the crankshaft 18. The section 34 is adjoined by a further cylindrical section 35 whose outer diameter is considerably greater. The crankshaft 18 lies, as per FIGS. 4 and 5, with the section 35 in a matched bore in the crankcase 11 into which the intake pipe 15 opens out. In the section 35, the crankshaft 18 has, formed from the rear end, a coaxial blind hole which forms a crankshaft duct or gas mixing duct 37. At the level at which the intake pipe 15 opens out, an inlet opening 38 in the wall of the section 35 is exposed which, in a certain rotational position of the crankshaft 18 (FIG. 5) connects the crankshaft duct 37 to the intake pipe 15 with the maximum cross section, but in the other rotational positions, largely blocks the connection (FIG. 4). The crankshaft 18 which is designed in this way forms a valve which is synchronized with the piston movement and which, over a predefined angular range of each rotation, permits the intake of air/fuel mixture from the carburetor via the crankshaft duct 37 and into the crank space 44.
As already described further above, the upward-traveling piston 24 generates a vacuum in the crank space 44, which vacuum, when the inlet opening 38 of the crankshaft 18 rotates into the region of the intake pipe 15, leads to an intake of the air/fuel mixture formed in the carburetor. The intake mixture flows in the axial direction through the crankshaft duct 37, then passes out into the crank space 44 and impinges on the opposite wall of the crankcase cover 20, is pre-compressed as the piston 24 moves downward and is then pushed out of the crank space via the lateral transfer ducts 32a-32c into the working space situated above the piston 24.
There are however various disadvantages in the described process. As indicated by the arrows and double arrows in FIG. 5, the mixture passing out of the crankshaft duct 37 impinges perpendicularly on the opposite wall of the cover 20, then rebounds in the opposite direction and thereby hinders the subsequent gas flow out of the shaft duct 37. This leads to a reduction of the mixture quantity available per working cycle and therefore to a degradation in engine power. The mixture flow which is aligned axially into the crank space 44 also has the disadvantage that the mixture, which simultaneously also serves to provide lubrication, only passes to a limited extent into the region of the rear ball bearing 16, so that the lubrication of the latter is not optimal. Finally, fundamentally only insufficient filling of the working space can be obtained with the mixture transport based exclusively on the piston movement.
It has been proposed in DE-A1-29 33 796 to provide, in a two-stroke model construction engine which has a plurality of cylinders (in a V-arrangement or star arrangement) and whose crankshaft is mounted on the crank cheeks, a rotary slide valve which allows the mixture to flow through to the cylinders in succession and has an axially aligned inflow and a radially orientated outflow. The rotary slide valve can, by means of the radially aligned outflow, control different ducts in the crankcase which are assigned to the different cylinders. The rotary slide valve is formed at the rear end of the crankshaft. The gas/fuel mixture is supplied to the rotary slide valve via a duct which runs in the axle and in which a throttle flap is arranged. The radial outflow of the air/fuel mixture out of the rotary slide valve is intended to generate a charging effect. Said known solution is designed for multi-cylinder engines. The solution cannot be used for conventional single-cylinder engines in which the carburetor and intake pipe are situated on the front side above the driveshaft and in which a starter device is in some cases installed on the rear side.
US-A1-2004/0079303 proposes the use, for small two-stroke engines which are charged with the air/fuel mixture via the crankcase, of a nozzle/diffuser combination (“nozzle diffuser”) which is arranged in the crankshaft axle. The nozzle diffuser is intended to have two effects: on the one hand, it should increase the turbulence of the mixture flow and thereby provide improved mixture and combustion. On the other hand, it should increase the speed and compression of the mixture and thereby provide a type of turbocharger effect which increases the power of the engine. In addition, radial ducts which lead outward from the nozzle diffuser can be provided in a counterweight. Said ducts are alleged to make the power of the engine more uniform. An increased improvement in power and a reduction in fuel consumption were observed when using a nozzle on its own and when using a nozzle/diffuser combination. A disadvantage of said solution, however, is that the narrowed cross section of the nozzle increases the flow resistance of the mixture between the carburetor and the crank space and therefore counteracts the suction action provided exclusively by the piston.